The Stovebolt Site Forums The Shop Area Engine Shop Is preignition the same as pinging?

Let’s talk about engine pinging.
I’ve been behind the wheel of a vehicle that had pinging with medium to hard acceleration. The sound it makes is unmistakable. Also, there is no power.
As Jerry has said in other threads, too much timing advance causes severe loss of power as well as engine damage.

I’ve learned that medium to hard acceleration causes the vacuum advance to drop back significantly. So I’m trying to sort out why pinging would occur at that time, because the ignition timing is reduced during moderate to hard acceleration. Right?

If a particular engine REQUIRES higher octane fuel, what would the symptoms be? My understanding is that lower octane fuel has a tendency to ignite too early in a high compression engine. I assume an engine needing higher octane fuel would ping on medium to hard acceleration. Would it also ping at highway cruising speeds?

Is this a generic question or specific to a certain vehicle? If a certain vehicle is it a stock Chevy six? Generally speaking pinging can be caused by a hot spot in one or more cylinders, too low octane for the compression ratio and advanced timing. There may be more causes I'm not aware of.

It’s a generic question.

As far as I can tell, I’m not experiencing any pinging. Since you asked, I have a rebuilt SBC 283 with power pack heads which would have fairly high compression. I’m running 87 octane fuel and I think it runs pretty good. I can accelerate in all gears without pinging.

There are two slightly different combustion conditions, "preignition" and "detonation". The way it was explained to me many decades ago was like this: "Preignition" is the beginning of a second flame front in the combustion chamber, one that was not originated by the spark plug. This can be caused by a variety of things, such as a hot spot like a glowing particle of carbon, a too-fast flame front travel that pushes the pressure wave to Diesel levels, poor combustion chamber design, or fuel that's too low octane. Additives that raise the octane rating slow down the burn rate of the fuel and turn the pressure wave into a steady push instead of a sharp punch. The more resistance to piston movement there is (lugging) the more likely preignition will happen. Running a 5% Diesel fuel mix can raise the octane rating of the fuel, sometimes as much as 5 points. That's one gallon of Diesel to 19 gallons of gas.

Once the second flame front begins, "detonation" is the noise made by the two flame fronts slamming together at supersonic speed. That's the "ping" that a driver hears. It's extremely destructive. Detonation has more power than dynamite, nitroglycerine, or any other explosive short of nuclear, when the weight of the reactive components is compared on a one to one basis. It produces extreme heat and pressure, to the point of melting piston material and spark plug electrodes, shattering porcelain spark plug insulators, and blowing holes in the tops of pistons, breaking rings, etc. By the time a driver hears that little ping or rattle, the pistons and other parts are getting sledgehammer blows, several times a second.

Retarding the vacuum advance under load is done to try to minimize preignition and detonation. If an otherwise good-running engine still pings and rattles on a long uphill pull at full throttle, try the next higher octane fuel, or retard the initial timing a little. That's why the GM engineers put the "octane selector" on stovebolt engines, so drivers could adjust the timing accurately if a bad tank of gas was encountered, then go back to the normal setting once that gas was used up.
Jerry

Let’s see if I have this right.
PRE-IGNITION = Squeeze the fuel molecules so tightly (high compression) that they ignite on their own (before the spark plug fires.

DETONATION = Ignition of fuel and expanding gases is early and fighting piston travel. They slam into each other making a pinging noise.

I think the rod bearings would also take a beating.

There's another factor that promotes detonation- - - -lean mixture. A mix that's heavy on oxygen and short on fuel becomes explosive and burns way too rapidly. That gets extremely destructive in a hurry. I had a tractor pull engine under load on a dyno one night at 6500 RPM and 700+ HP when an intake manifold bolt loosened up and caused one cylinder to go lean. I heard a little rattle, and before I could back off on the load and chop the throttle, that cylinder had burned a hole in the piston the size of a half dollar- - - -in maybe 10 seconds or less. Lean mixtures and early timing have destroyed a lot of engines.
Jerry

But you would HEAR IT, right?

By the time you hear the noise, detonation has been going on for quite some time. Think about all the cast iron, sheet metal, and insulating material that sound has to penetrate before the driver actually becomes aware of it. Add a little middle-aged high frequency hearing loss to that situation, and it's possible some drivers won't be able to hear even severe engine noises before they start doing damage to internal parts. Chasing that mythical "ideal" timing advance setting is pretty much of a fool's mission! I've proved that to my own satisfaction on dozens of dyno runs. A tiny change in ambient temperature, gasoline quality, or relative humidity can throw even the most complicated of timing calculations a curve. We ran a series of dyno pulls on a round track engine one morning and had the engine dialed in well. Then we went to lunch and a thunderstorm came through the area, which changed the humidity. On the afternoon run, we lost 12 HP. "Your mileage may vary!"
Jerry

I have overdrive. If there is any chance of detonation I would think it would be in overdrive.

Definitely- - - -anything that promotes engine lugging- - - -failure to accelerate with increased throttle, is where detonation is most likely to happen. Overdrive is great, as long as it's only used appropriately. A dash-mounted vacuum gauge will educate a driver about when it's appropriate to use a higher gear ratio, and when the engine is getting out of the torque band and performance and gas mileage starts dropping due to too tall gearing.
Jerry

Compression causing ignition is called DIESEL. IMHO not possible under 15:1 CR.

My compression isn’t anywhere near that. I’m not hearing any pinging at any speed or RPM (being careful not to lug the engine of course) so I am glad that it seems to be happy with 87 octane.

I've seen 9:1 engines Diesel- - - -just shut the ignition switch off at around 1K RPM or a little faster.
Jerry

A number of engines, including GM ones, use an idle stop solenoid or high idle solenoid. It closed the throttle further than normal idle speed when the key was turned off to prevent dieseling.

Fuel injection solved the problem for good- - - - -no fuel to support dieseling once the ignition switch was shut off.
Jerry

This is not quite accurate.

As Jerry mentions, detonation is caused by the heat of combustion raising the combustion chamber temperature above the auto-ignition point of the fuel, which causes unburned pocket(s) of air/fuel mixture to self ignite. The pressure waves from these flame fronts collide, causing a rapid increase in cylinder pressure. These pressure spikes actually cause the cylinder walls to resonate, resulting in the audible “pinging” sound heard from the engine.

Peak pressures during detonation occur after TDC, since the pressure/temperature rise that initiates it is a result of the combustion event. Severity of detonation can range from mild (annoying) to severe (engine damage).

One way to control detonation is to use fuel with higher octane. The octane rating of fuel is directly related to the fuel’s resistance to self-ignite. The higher the octane rating, the higher the resistance to self-ignite, which means higher resistance to detonation. Anyone old enough to remember black and white TVs or 0.30/gallon gas, knows that the go-to octane enhancer used to be tetraethyl lead. Lead was phased out of fuels in the 1970’s to support the use of catalytic converters, and for environmental concerns. Ethanol is frequently used in modern gasolines to increase octane rating.


Jerry, you mention that you use diesel fuel to increase octane. Since diesel engines are designed to ignite the air/fuel mixture using the heat of compression, diesel fuels have extremely low octane ratings. Their self ignition temps are low by design. I’m struggling to understand the science behind how adding diesel fuel can increase octane rating of gasoline. Please explain.

Some of the factors related to detonation are:

- Combustion chamber temp/press
- Spark advance (which affects combustion chamber temp/press)
- Compression ratio (ditto)
- Air/fuel mixture (ditto)
- Inlet air temperature (ditto)
- Engine load (ditto)

Modern engine management systems monitor for detonation using a detonation (aka “knock”) sensor. When detonation is detected, the engine controller can take corrective action to mitigate detonation, such as retarding spark advance, reducing throttle, or changing the air/fuel mixture. As was also previously mentioned, the time honored demonstration of this was to tap on the side of the block with a hammer while watching spark advance - either with a timing light or scan tool. The hammer hit causes the block to resonate at a frequency close enough to detonation to cause the engine controller to react.

In contrast, preignition, is caused by an ignition source other than the spark. Sources include: hot spots in the combustion chamber due to carbon build up, a spark plug that is too hot due to incorrect heat range selection, sharp edges on the cylinder head, valves, etc,

The bad thing about preignition is that as the name implies, it occurs well before the piston has reached TDC, causing very rapid pressure and temperature increases in the combustion chamber. It often occurs so quickly, that the engine damage is done before the driver is even aware that it is happening.

BTW, preignition is not just something to be worried about with our engines. A relatively new phenomenon called “Stochastic Preignition” is currently a source of concern among automakers designing small displacement, direct inject, turbocharged engines.

Stochastic Detonation

As a quick sidebar to detonation, in the summer of 1982, I was at a party talking to a very attractive young brunette, who had just got her 1981 Olds Cutlass back from the dealer for the umpteenth time. The complaint was that the engine often made a “rattling noise” when she stepped on the gas. Each time she got it back, it seemed like the car ran worse. The dealer finally advised her to start running premium fuel, because of the “high performance” Olds V8 under the hood — which was beyond stupid, because as anyone who worked on the cars of that era knows, the compression ratio of that Olds 307 was lower than low back then.

I told her that I would look at it, and if I fixed it, she could buy me dinner.

I quick check of the ThermAC system (the thermostatic air control that preheats incoming air to the carburetor when the engine is cold) showed that the vacuum control valve was faulty, causing the system to provide heated air to the carb at all times. This super heated air, entering the combustion chamber, was enough to induce detonation under certain engine loads. I replaced the valve, and the system worked fine. A check of the ignition timing, showed that in an effort to fix the problem, the nobs at that dealer had uber-retarded the spark causing the engine to lose power and fuel economy. I reset it to factory specs and the car ran great.

I got my dinner. As of last May, we have been married 37 years.

Best Regards…

p.s. A lot of the above information was gleaned from this source. It an older textbook, but it has a lot of interesting information related to internal combustion engines and related topics. Plus, it is a freebie download.


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My understanding of the advantages of adding Diesel to the gas might be in error, but I've seen a definite power increase on numerous dyno runs with doctored fuel, and less of a tendency to spark knock on long uphill pulls on repeated road tests. One theory is that the Diesel slightly increases the specific gravity of the fuel, and slows down the burn rate slightly- - - -the same thing adding an octane booster does. I don't fully understand the physics behind it, but the improvement can be documented- - - -repeatedly! Since Diesel has slightly more BTU's per gallon, I've also seen a marginal improvement in gas mileage when running Diesel-boosted fuel.
Jerry

The pre detonation is actually the fuel molecules breaking apart under the force of compression. Higher octane fuels resist this breaking apart under high compression long enough for the spark to ignite the intact fuel molecule.

Just who invented this imaginary term "Pre-Detonation"? There's "pre-ignition" which is a flame front that begins somewhere other than at the tip of the spark plug, and "detonation" which is uncontrolled, possibly explosive burning of the fuel/air mixture. How did those terms get combined? In over 60 years of tinkering with engines and spending decades teaching the mechanical trade, I've never heard that one before!
Jerry

My bad. Typing in a hurry. I meant to type pre ignition.

I used the term PREDETONATION in the thread title before I knew it wasn’t the correct term. I will NEVER use it again (unless I’m heard mumbling in my sleep).

That's probably why I typed it! It was in my subconscious mind!

No harm- - - -no foul! Since it seems that combustion chamber dynamics are often misunderstood, when this thread pops up on somebody's web search five years from now, at least they'll be reading the correct terms. "What's the difference in love and the internet"?- - - - - -The internet is forever!
LOL!
Jerry

Pinging and pre-ignition are not the same thing, although pre-ignition can cause pinging. Pre-ignition occurs when the air fuel mixture ignites before the spark plug fires. This can be caused by hot spots forming in the combustion chamber (for example due to carbon glowing red hot) poor fuel quality (low octane), excessive cylinder head temperatures, etc. Note that pinging can also be caused by excessively advanced ignition timing, in which case it is notcaused by pre-ignition but by incorrect timing. Pre-ignition if sustained may cause engine damage and should be investigated.

Using the term detonation opens a door to another discussion...

Steve